1. Field of the Invention
The present invention relates to an electronic fuel injection apparatus for a diesel engine, and in particular to an electronic fuel injection apparatus for a diesel engine which determines a target injection quantity based on a driving state of a vehicle.
2. Description of the Related Art
For a conventional electronic fuel injection apparatus for a diesel engine, not only an injection quantity and injection timing but also an injection pressure is important as a control parameter for an injector. Various electronically controlled apparatuses which can control the injection pressure have been thus developed.
For example, the published Japanese translation No. 6-511526 of PCT international publication for patent applications discloses a hydraulically (oil) actuated injector. In this injector, the pressure action of hydraulic fluid (engine oil) is controlled through an electronic device such as an electromagnetic valve which is internally provided, thereby controlling the fuel injection quantity and timing.
Namely, a fuel chamber formed within the injector is preliminarily supplied with fuel of relatively low pressure. Then, the fuel injection is performed by a controller which energizes the electromagnetic valve or opens the valve provided in a route through which high-pressure oil pressurized by a high-pressure oil pump to be accumulated within an oil rail is supplied to a pressured surface of a pressurizing plunger in the injector from the oil rail to actuate or give strokes to the pressurizing plunger.
Also, in this injector, by controlling the pressure of the hydraulic fluid (pressure within the oil rail) through a flow control valve provided in the high-pressure oil pump, the fuel injection pressure may be controlled.
Therefore, the controller calculates a target injection quantity, a target injection timing, and a target injection pressure corresponding to a driving state of the engine, thereby determining a duration and a timing for energizing the electromagnetic valve and the duty ratio of the flow control valve based on respective target values.
Besides, an electronic fuel injection apparatus which uses a fuel-actuated injector (injection valve) such as described in the Japanese Patent Publication No. 4-19381 is also well known. This injector has a pressure control chamber formed at the discharging side of a needle valve preliminarily supplied with high-pressure fuel, which is then leaked for the fuel injection in opposition to the above-mentioned published Japanese translation No. 6-511526.
Namely, the high-pressure fuel is pressurized by the high-pressure fuel pump and supplied to the tip of the needle valve and the above-mentioned pressure control chamber through an accumulator. The fuel injection is performed by energizing or opening the electromagnetic valve provided in a leak passage in the pressure control chamber.
In view of the target injection quantity among the above-mentioned control parameters, the injector described in the above-mentioned published Japanese translation No. 6-511526 may encounter a large fall or depression of an actual injection quantity (see a characteristic Q1 in FIG. 1C) with reference to the target injection quantity, and an unstable engine speed or engine stall at worst in a short time after engine start at low temperatures, i.e. immediately after the transition from the cranking mode to the completed explosion mode. This is because low temperatures also make the oil temperature low so that the oil becomes so highly viscid as not to pass through the opening of the electromagnetic valve whereby the electromagnetic valve is closed before the pressurizing plunger have its full stroke, leading to such a fall of the actual injection quantity.
In order to eliminate such a fall of the actual injection quantity, the published Japanese translation No. 6-511526 detects the oil temperature with an oil temperature sensor provided in the oil rail and corrects the target injection quantity particularly in the increasing direction at low temperatures or the energizing time. However, with only such corrections on oil temperature, the actual injection quantity corresponding to the target cannot be obtained in a short time after engine start at low temperatures as mentioned above.
This is because the rising rates of the actual oil temperature (fuel temperature) differs between the insides of the oil rail (or the accumulator) and the injector.
Namely, while the oil supplied to the injector is theoretically to be discharged out of the injector at each single injection, a considerable amount of the oil actually remains. Also, since the electromagnetic valve is actuated at a high speed inside the injector, the residual oil is stirred and the temperature rises quickly. However, the oil within the oil rail (in-rail oil) is gradually raised in temperature by the circulation thereof around the injector or parts of the engine.
Therefore, as shown in FIG. 1D, while a temperature Ti of the oil within the injector quickly rises in a short time after engine start, a temperature Tr of the oil within the oil rail hardly changes. Therefore, the controller assumes that the temperature is low in a short time after engine start and corrects the target injection quantity in the increasing direction.
Now assuming that for the fall of the actual injection quantity in a short time after engine start at low temperatures the target injection quantity is corrected in the increasing direction according to an oil temperature correction characteristic Qcrct1 shown by a solid line in FIG. 1B, namely the oil temperature correction quantity is set to be corrected after the oil temperature within the oil rail starts rising, the actual injection quantity in a short time after engine start becomes insufficient as shown by a characteristic Q2 in FIG. 1C, thereby causing the engine stall. Also, assuming that the target injection quantity is largely increased according to an oil temperature correction characteristic Qcrct2 as shown by a dotted line in FIG. 1B, the correction quantity becomes excessive as the oil temperature rises within the injector as shown by a characteristic Q3 of a dotted line in FIG. 1C and the fuel injection exceeds the target quantity, thereby causing the engine speed to abnormally rise or generate a large amount of white smoke.
Such problems may also arise in a fuel-actuated injector as described in the Japanese Patent Publication No. 4-19381 as well as the published Japanese translation No. 6-511526.